Source Sans vs Inter: Which for UI?

Two typefaces currently dominate the conversation about fonts for user interfaces: Source Sans 3 from Adobe and Inter from Rasmus Andersson. Both are free, both are widely adopted, both prioritize legibility in interface contexts, and both are technically sophisticated — Inter as a variable font from the start, Source Sans 3 following with its own variable font update. Yet they arrive from fundamentally different design philosophies, and those differences produce meaningfully different results in real interfaces.

This comparison examines both typefaces across the dimensions that matter for UI typography: their origins and design intent, their visual character, their technical metrics, their behavior across rendering environments, and the specific interface contexts where each genuinely outperforms the other. Both deserve their popularity — understanding why each earns it is the point.

Design History and Philosophy

Source Sans 3: Adobe's Contribution to Open Type

Source Sans was designed by Paul Hunt at Adobe and released in 2012 as Adobe's first open-source typeface — the first member of what would become the Source family (Source Serif, Source Code Pro, Source Han). Hunt's design brief was to create a humanist sans-serif that would be highly readable in user interfaces and body text, drawing on the humanist sans traditions of designers like Frutiger and Gill while optimizing the result for the specific demands of digital display.

The original Source Sans Pro was widely adopted and became one of the most-used open-source typefaces on the web. In 2022, Adobe released Source Sans 3, a substantially updated version of the family that added a variable font format, extended the character set significantly, refined the existing glyphs, and improved OpenType feature support. Source Sans 3 is not merely a version bump — it is a significant upgrade that makes the family considerably more capable as a design system typeface.

Hunt brought careful humanist sensibility to Source Sans. The letterforms draw explicitly on the Venetian and humanist sans traditions: the double-story "a" and "g" maintain legibility at small sizes, the apertures in letters like "c" and "e" are open to prevent letters from closing up on screen, and the overall rhythm of the letterforms reflects hand-drawn origins even in a modern sans context. Source Sans has a warmth and personality that pure geometric sans faces lack — it is designed for people, not for machines.

Inter: Purpose-Built for Screen UI

Inter was created by Rasmus Andersson, a Swedish designer and software engineer who spent years at Facebook and Figma, and released publicly in 2017. The design brief was unlike any other typeface project: Andersson's explicit goal was to create the optimal typeface for reading and understanding text in computer interfaces. He was not drawing from historical tradition; he was responding to a technical problem with a technical solution informed by design craft.

Andersson spent years iterating on Inter before its public release, testing it against the specific rendering challenges of computer screens — particularly at the small sizes where UI text actually gets read: 11–14px in labels, tooltips, form fields, navigation, and table data. He optimized for legibility at these small sizes on both macOS and Windows, and he built Inter as a variable font from the beginning, allowing smooth interpolation across its weight axis rather than requiring discrete weight files.

The design philosophy of Inter is sometimes described as "neo-grotesque" or "rational," but that undersells the care in its humanist details. Andersson made specific decisions to maintain open, readable letterforms: the crossbars on lowercase letters like "t" and "f" are positioned to avoid closing apertures; the "l" (lowercase L) has a distinctive curved tail to distinguish it from "I" (capital I) and "1" (numeral one) — a critical disambiguation for UI text where these characters appear frequently in usernames, codes, and identifiers.

Visual Differences at a Glance

Spacing and Letterspacing

Inter runs wider than Source Sans 3. The glyphs are slightly broader, and Inter's default letter-spacing (tracking) is calibrated for screen reading at small sizes — slightly more generous than Source Sans 3's defaults. This means that at identical font sizes and default tracking settings, text set in Inter will take up more horizontal space. In constrained UI layouts — sidebar navigation, mobile interfaces, form labels — this difference can affect how many characters fit per line.

Source Sans 3's slightly narrower proportions make it more efficient in space-constrained contexts. Where Inter might need an 11px minimum for a readable navigation label, Source Sans 3 can read comfortably at 10px. This is a genuine practical difference for dense UI designs.

Character Disambiguation

Inter was explicitly designed with character disambiguation as a priority. The lowercase "l" (ell), uppercase "I" (eye), and numeral "1" are clearly distinguishable from each other — the lowercase l has a distinctive curl, and the capital I has serifs in Inter's default configuration. Source Sans 3 draws the lowercase l with a vertical stroke without a distinctive tail, and the capital I without serifs, making the two somewhat similar. In most body text contexts this is not a problem, but in UI contexts where users read codes, usernames, serial numbers, and identifiers, Inter's disambiguation is a material legibility advantage.

Italics and Obliques

Source Sans 3 includes true italics across its weight range — the letterforms change structure, not merely inclination. The italic forms are graceful and maintain the typeface's humanist character. Inter's italic variant was added later in the project and is more conservative — primarily obliqued forms with some structural changes. For UI purposes, italics are used rarely enough that this difference is minor, but in editorial UI contexts (blogs, documentation, long-form product content), Source Sans 3's italic quality is superior.

Metrics Comparison

Both typefaces are available as variable fonts through Google Fonts, which is the recommended loading method for either.

Inter covers a weight range of 100–900 on its wght axis, making the complete weight spectrum available within a single font file. Source Sans 3 covers the same range (100–900) on its variable axis. For design systems that need a complete weight range, both provide equivalent coverage.

The x-height comparison is revealing: Inter has a notably large x-height — the lowercase letters are tall relative to the capitals, maximizing legibility at small sizes. Source Sans 3 also has a generous x-height, though slightly less extreme than Inter. At equivalent sizes, Inter lowercase appears slightly larger and heavier, which is the desired effect for UI contexts where density and legibility at small sizes matter.

/* Inter — UI-optimized settings */
body, button, input, select {
  font-family: 'Inter', system-ui, sans-serif;
  font-size: 0.875rem;    /* 14px — Inter holds up well here */
  line-height: 1.5;
  font-weight: 400;
  /* Inter's built-in CV variants for disambiguating l/I/1 */
  font-feature-settings: 'cv01', 'cv03', 'cv04';
}

/* Source Sans 3 — UI settings */
body, button, input, select {
  font-family: 'Source Sans 3', system-ui, sans-serif;
  font-size: 0.875rem;
  line-height: 1.5;
  font-weight: 400;
}

Inter ships with a rich set of OpenType features, including numerous character variants (cv01–cv99) that allow customization of individual letterforms. The "cv01" feature activates one-story lowercase "a"; "cv05" provides alternative lowercase "l"; "ss01" enables contextual alternates including the full-serif capital I. For design systems that want to fine-tune Inter's character disambiguation, these features provide precise control.

Source Sans 3 also includes substantial OpenType feature support, including true small capitals, proportional and tabular figures, and a range of numeral styles. For interfaces that need typographically sophisticated numerals in financial or data contexts — tabular alignment in tables, oldstyle figures in flowing text — Source Sans 3's OpenType feature set is more developed for these classical typographic needs.

Rendering Across Platforms

Inter was designed with Windows rendering as an explicit consideration. Andersson tested and iterated on Inter's hinting behavior on Windows ClearType extensively, and the result is a font that renders with exceptional clarity on Windows at small sizes. If your application has significant Windows user traffic, Inter's rendering quality is a meaningful advantage. The large x-height and robust stroke weights survive sub-pixel rendering without degrading.

Source Sans 3 also renders well on Windows — Adobe's font engineering teams brought considerable expertise to its hinting — but Inter's rendering at small UI text sizes on Windows is the benchmark against which other UI typefaces are measured. On macOS with its high-fidelity Quartz rendering, the gap between the two narrows significantly, and both fonts look excellent.

On mobile platforms, both fonts perform well. iOS and Android both use high-DPI displays where hinting quality is less critical, and the variable font format for both allows fine-tuned weight selection for different screen contexts. Inter is the default interface font in Figma, which has contributed to its widespread adoption — designers who compose in Figma and export to web find that Inter produces minimal discrepancy between design and implementation.

System font stacks are worth considering as an alternative: macOS uses San Francisco, Windows uses Segoe UI, Android uses Roboto. If you want your interface to feel native on each platform without loading a custom font, the system font approach described in CSS font stacks is an alternative. But for consistent cross-platform appearance, both Inter and Source Sans 3 are superior to relying on system fonts.

Best Use Cases for Each

When Inter Excels

Inter is the optimal choice for any interface where text is read at small sizes, where character disambiguation matters, and where Windows rendering quality is important. Application UIs, developer tools, admin dashboards, data tables, form-heavy interfaces, and coding environments are all natural homes for Inter. Figma's use of Inter across its own interface is perhaps the most prominent endorsement — a design tool that uses Inter to compose design systems naturally demonstrates the font's UI capabilities daily to millions of designers.

Inter also excels in design systems at scale. Its variable font range, comprehensive OpenType features, extensive character set (it covers nearly all Latin-script languages plus Cyrillic and Greek), and freely available Figma components make it the lowest-friction choice for a design system that needs to scale to multiple teams and products. The Inter font guide covers its variable font capabilities and design system integration in depth.

For developer tooling, documentation platforms, and productivity applications where precision and clarity are paramount, Inter's disambiguation features and small-size legibility make it the clear choice over alternatives like Roboto or IBM Plex Sans.

When Source Sans 3 Excels

Source Sans 3 is the better choice when the interface contains significant amounts of body-length reading text, when the brand requires more warmth than Inter's rational character provides, or when the project also has print or document output components that would benefit from a typeface with stronger print heritage.

Adobe's own applications use Source Sans (in its various versions) across Creative Cloud, Acrobat, and other products — which reflects a deliberate brand choice to project warmth, approachability, and craft alongside technical precision. If your product design language wants to evoke those qualities rather than pure rational efficiency, Source Sans 3's humanist character serves the brief.

Source Sans 3 also wins on character set sophistication. The OpenType feature set for fine typography — small capitals, multiple numeral styles, ligatures, case-sensitive punctuation — is more comprehensively implemented than Inter's. For applications that care about typographic quality in ways beyond pure legibility at small sizes, Source Sans 3's feature depth rewards investment. The comparison with IBM Plex Sans is also worth reading if you need a typeface with strong corporate identity and comprehensive OpenType features.

The Verdict: When to Choose Which

The decision comes down to your primary use case and the qualities you most need to optimize.

Choose Inter when your interface is text-dense, when character disambiguation matters (codes, identifiers, usernames), when Windows rendering quality is critical, when you need maximum legibility at 11–14px, or when your team already works in Figma and wants design-to-development consistency without friction. Inter has earned its position as the default choice for serious UI typography through consistent performance across all these criteria.

Choose Source Sans 3 when your interface includes substantial reading-length text, when warmth and approachability are part of the brand requirement, when you need sophisticated typographic features (tabular numerals, small caps, oldstyle figures), when print or PDF output matters, or when a more humanist design character is preferable to Inter's rational coolness. Source Sans 3 is the choice for teams who want their product to feel crafted rather than merely optimized.

Both are unambiguously better choices for UI than aging alternatives like Helvetica Neue, Arial, or the original Source Sans Pro. For teams building a comprehensive design system with a thoughtful typographic foundation, the choosing a primary font for a design system guide provides a framework for evaluating this decision alongside all the other system-level considerations it touches.